1. Field of the Invention
The present invention relates to a NOx reduction device for diesel vehicles.
2. Description of Related Art
FIG. 1 is a schematic diagram illustrating a part of a conventional exhaust device for diesel vehicles, FIG. 2 is a graph illustrating the relationship of the temperature of exhaust gas with respect to time at an inlet of a particulate filter, and FIG. 3 is a graph illustrating the NOx purifying ratio of a NOx catalyst with respect to temperature.
The exhaust device for diesel vehicles has a function of removing nitrogen oxides, collectively referred to as NOx, and a function of removing soot when a predetermined volume or more of soot has accumulated.
The exhaust device for diesel vehicles is equipped with a Lean NOx Trap (LNT), also referred to as a de-NOx catalyst, and a recyclable Diesel Particulate Filter (DPF) for removing particulate matter such as soot.
As can be seen in FIG. 2, in the regeneration of the DPF, the temperature at the inlet of the DPF is required to be about 600° C. In order to obtain this temperature, a portion of the fuel being fed into the engine is introduced into the exhaust device to raise the temperature through oxidization, caused by the LNT.
A Hydrocarbon-Selective Catalyst Reduction (HC-SCR) catalyst is also provided, which removes NOx using hydrocarbon. For this, a post injector, which injects a necessary amount of fuel containing hydrocarbon, is provided.
Thus, the HC-SCR catalyst is not provided in a conventional exhaust device, which does not have a post injector.
As can be seen in FIG. 2, in the regeneration of a Diesel Fuel Catalyst (DFC), the temperature at the inlet of the DFC is about 600° C. As can be seen in FIG. 3, in the generation of the DFC at a temperature of 600° C. or more, the LNT generally does not ensure a NOx purifying ratio.
Accordingly, in the regeneration of the DPF, the conventional exhaust device for diesel vehicles shown in FIG. 1 disadvantageously emits NOx as it is.
Furthermore, in the regeneration of the DPF, Exhaust Gas Recirculation (EGR) is generally turned off in order to stably purify soot. At this time, the LNT does not efficiently remove NOx whereas the amount of NOx emissions increases up to 5 to 20 times more than normal.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.